The present invention relates to a method of forming a sealed seam between a tear-off film and a packaging member.
A large number of consumer products are offered in packages that are closed by a tear-off film. For example, cup-shaped containers are sealed with the film made of aluminum, plastics or a composite material. Typically, the sealing is achieved by introducing the packaging container and the film superposed thereon between two dies and compressing them between these dies, so that the meltable sealing layer of the film can form a welding-type joint with the edge of the packaging member.
In this sealing step, the upper die may for example be moved from above against the lower die that supports the packaging member, or the lower die is lifted against the upper die. A combination of such axial movements of the dies is also possible, as been described for example in EP 1 990 281 A2. In what follows, the terms upper die and lower die shall not be construed as limiting the orientation of the dies in space, but are merely motivated by the fact that the packaging member is conveniently arranged in the lower die. Moreover, the term “packaging member” shall designate both, packaging containers to be sealed, such as cups or the like, as well as parts thereof. For example, it is common practice that only a part of the package, e.g. an intermediate ring is sealed with a film in the way described above, and this intermediate ring is then joined to the rest of the package by crimping or any other method in order to form a container.
In the sealing step, a complete and continuous tight peripheral sealed seam is formed in the peripheral portions of the film and the packaging member. This sealed seam must be strong enough to reliably close-off the package. In a known method, for example, a first sealing step is followed by one or more further sealing steps in which the width of the sealed seam is increased or the bonding strength is improved by compressing the film and the packaging member once again. On the other hand, measures which improve the strength of the sealed seam, such as a widening of the seam, make the tear-off process more difficult. It is therefore desirable to form the seam in such a way that, on one hand, it can withstand a high internal pressure and, on the other hand, the necessary tear-off forces are reduced.
In an initial phase of an opening process, the largest tear-off forces will generally occur at the tear-off strip. When, for example, an arcuate seam is exposed to tearing forces from outside, the length of the tear-off front increases rapidly in the direction transverse to the tear-off direction, i.e. over the width of the package. It is possible to reduce the tear-off forces by means of notch-like or arcuate cut-outs in the outer edge of the welded seam, as has been disclosed for example in EP 0 344 340 A1 and CH 680 422 A5. The solutions disclosed in these documents concentrate mainly on the reduction of the forces in the initial phase of tearing-off the seam and not on a reduction of the maximum tear-off force. The latter is frequently increased by bulges of the sealing material at the edges of the seam, which bulges result from the effect that, when the packaging element and the film are compressed, sealing material is displaced towards the inside and towards the outside of the package. In the vicinity of the bulges, the sealing strength is particularly high and must unfortunately be overcome in the very moment in which the tear-off front reaches the maximum length across the sealed seam. The problems resulting from this effect are neither addressed nor solved in the prior art cited above.